Heat treatments to improve magnetic properties of ferro-magnetic materials are known in the art. For instance, U.S. Pat. No. 2,569,468 ("Gaugler") discloses a treatment wherein ferro-magnetic material is subjected to severe cold reduction sufficient to produce grain-orientation followed by annealing in a magnetic field to produce rectangular hysteresis loops. The materials treated according to the method of Gaugler include 50% Ni--Fe alloys and commercial grades of silicon steel. In one embodiment, a sheet of 50% Ni--Fe alloy is slit into tape which is insulated and wound into spiral cores, the cores are mounted in an annealing pot, the pot is inserted into a furnace at 1000.degree.-1150.degree. C., the cores are heated for two hours and rapidly cooled by withdrawing the pot from the furnace. The cores can be given a second anneal in an atmosphere of pure hydrogen above the magnetic transformation point (Curie temperature, T.sub.c) at approximately 500.degree. C. and the cores are cooled slowly in a strong magnetic field of approximately 87 Oersteds. During the second anneal, the cores are suspended or supported in spaced relation within a pot by a suitable medium such as aluminum oxide. Hydrogen is admitted into the pot by way of suitable ports.
It is also known in the art to magnetic anneal amorphous metal alloys to tailor the magnetic properties thereof for specific product applications. A number of magnetic amorphous metal alloys are produced on a commercial scale by Allied Corp., now Allied-Signal, Inc. located in Morristown, N.J. and are marketed under the "METGLAS" trademark. For instance, magnetic annealing treatments for amorphous metal alloys are disclosed in U.S. Pat. No. 4,081,298 ("Mendelsohn"), U.S. Pat. No. 4,262,233 ("Becker"), U.S. Pat. No. 4,268,325 ("O'Handley"), U.S. Pat. No. 4,649,248 ("Yamaguchi"), U.S. Pat. No. 4,668,309 ("Silgailis I"),, U.S. Pat. No. 4,769,091 ("Yoshizawa"), U.S. Pat. No. 4,809,411 ("Lin"), and U.S. Pat. No. 4,877,464 ("Silgailis II").
Amorphous metal alloys are typically made by rapid quenching from a melt in a continuous casting process. When the cooling rate is high enough (up to millions of degrees per second, depending on the alloy) atomic mobility decreases too rapidly for crystals to form, and no long-range atomic order develops. Amorphous metal alloys containing ferrous or other magnetic metals exhibit increased magnetic permeability because of the absence of long-range order. The amorphous metal alloys typically include metalloid atoms IIIA, IVA, and VA elements such as boron, carbon and phosphorous. The function of the metalloids is to lower the melting point, allowing the alloy to be quenched through its glass transition temperature (T.sub.g) rapidly enough to prevent formation of crystals.
The METGLAS alloys include iron-based alloys with additions of boron and silicon such as Alloy Nos. 2605 TCA, 2605 SC, and 2826 MB as well as a cobalt-base alloy (Alloy No. 2714A). The iron-based alloys offer high saturation induction, meaning they can produce very strong magnetic fields. These strong fields are associated with easily-aligned magnetic domains, clusters of like-magnetized atoms.
The major application of iron-based amorphous alloys is for transformer cores, in which they reduce energy lost by the core. Core losses in conventional alloys are associated with Eddy currents, contaminants, and with rotating domains and moving domain walls, which must overcome constraints imposed by the crystalline structure. The lack of this structure and absence of oxide inclusions in amorphous metals reduce these losses. Compared to conventional silicon steel, amorphous alloys used as core material in transformers can reduce wasted energy by as much as 70%.
Amorphous metal alloy ribbons typically have a thickness of only 25 to 40 microns. Accordingly, many layers of material are required to build up a given thickness of winding or lamination.
Of the foregoing U.S. Patents, Mendelsohn discloses that rapid quenching associated with glassy metal processing tends to produce non-uniform stresses in as-quenched filaments of the alloys. Mendelsohn discloses that heat treating tends to relieve these stresses and results in an increase in the maximum permeability. Mendelsohn discloses a heat treatment for glassy magnetic alloys of nominal composition Fe.sub.40 Ni.sub.40 P.sub.14 B.sub.6 (all subscripts herein are in atom percent). The heat treatment is performed at a temperature no higher than 350.degree. C. The crystallization temperature (T.sub.x) of the alloy is about 375.degree. C. After heating, the alloy is cooled through the Curie temperature T.sub.c (about 247.degree. C.) at a cooling rate no faster than about 30.degree. C./min. The heat treatment can be carried out in the absence of an externally applied magnetic field or by employing a magnetic field of about 1 to 10 Oe during cooling through the Curie temperature. Mendelsohn discloses that the amorphous metal alloy must be substantially glassy, that is, at least about 80% of the alloy as quenched should be glassy. The terms "glassy" and "amorphous" are used interchangeably in the art.
Becker discloses that ferrous amorphous alloys can be processed by magnetic annealing to develop useful AC permeabilities and losses. Becker discloses that ribbons of a ferrous amorphous alloy are heated in a temperature and time cycle which is sufficient to relieve the material of all stresses but which is less than that required to initiate crystallization. For instance, the sample may be either cooled slowly through its Curie temperature T.sub.c, or held at a constant temperature below its Curie temperature in the presence of a magnetic field. As an example, Becker discloses that toroidal samples were made by winding approximately 14 turns of MgO-insulated ribbon in a 1.5 centimeter diameter aluminum cup and 50 turns of high temperature insulated wire were wound on the toroid to provide a circumferential field of 4.5 Oe for processing. The toroids were sealed in glass tubes under nitrogen and were heat treated for two hours. The alloy had the nominal composition of Ni.sub.40 Fe.sub.40 P.sub.14 B.sub.6.
O'Handley discloses annealing of a magnetic glassy metal alloy sheet in a magnetic field. O'Handley discloses that the alloy may include a minor amount of crystalline material but the alloy should be substantially glassy in order to minimize the danger of growth of crystallites at high temperature (above 200.degree. C.), which would lead to a significant loss of soft magnetic properties. O'Handley discloses that alloys such as Fe.sub.40 Ni.sub.40 P.sub.14 B.sub.6 and Fe.sub.80 B.sub.20 develop exceptionally high permeability as quenched during their processing. The anneal of O'Handley is performed at an elevated temperature below the glass transition temperature T.sub.g and above about 225.degree. C. O'Handley defines the glass transition temperature T.sub.g as the temperature below which the viscosity of the glass exceeds 10.sup.14 poise. The alloy is cooled at a rate of 0.1.degree.-100.degree. C./min. and the annealing is discontinued when the temperature is 100.degree.-250.degree. C., preferably 150.degree.-200.degree. C. O'Handley discloses that the annealing treatment is applicable to wrapped transformer cores comprised of a coiled tape and ring-laminated cores comprised of a stack of circular planar rings. In a specific example, tape-wound toroids of Fe.sub.40 Ni.sub.40 P.sub.14 B.sub.6 were annealed at 325.degree. C. for 2 hours and cooled at a rate of 1.degree. C./min. in a 10 Oe circumferential field.
Yamaguchi discloses an annealing furnace for annealing magnetic cores, such as magnetic cores formed of a coiled strip of an amorphous metal alloy having a very thin thickness. Yamaguchi discloses that a conventional method of annealing magnetic cores includes winding a coil around the magnetic core for magnetizing the core, charging the core into an annealing furnace together with the magnetizing coil, evacuating gas in the furnace, introducing inert gas into the furnace and raising the temperature of the furnace to anneal the core in a magnetic field generated by the magnetizing coil. The annealing furnace of Yamaguchi allows the cores to be annealed in a magnetic field in a continuous manner.
Silgailis I and II each disclose a method of magnetic annealing amorphous metal in molten tin. The magnetic annealing is performed by applying a saturation field to the core while it is immersed in a liquid whose temperature is in the range between 0.7-0.8 T.sub.g (the glass transition temperature of the alloy). After annealing, the core is removed and rapidly cooled by immersion in a cooling fluid such as a slurry of acetone/dry ice at minus 78.degree. C. To prevent penetration of molten metal, the core can be coated before immersion in the hot liquid with a material which will eliminate adhesion of the liquid to the core. Alternatively, the core can be wrapped in a protective wrapper such as fiberglass, polyamide film (e.g., "KAPTON" polyamide film), metal foil, etc. In one example, a core wound from amorphous ribbon of Fe.sub.78 B.sub.13 Si.sub.9 was coated with "NICROBRAZ" dewetting agent and placed into a bath of molten tin-based solder at 400.degree. C., as a saturation magnetic field was applied to the core. When the temperatures of the bath, core skin, and core center were within about .+-.5% of the soak temperature, the core was held at that temperature for about 4-8 minutes after which the core was removed from the bath and cooled to room temperature in a slurry of acetone/dry ice at minus 78.degree. C.
Yoshizawa discloses a process of heat treating a magnetic core comprised of an amorphous metal alloy ribbon formed into a toroid. The process includes heating the core to a temperature above the alloy's Curie temperature (T.sub.c), slowly cooling the core through the Curie temperature in a DC or AC magnetic field at a rate of 0.1.degree.-50.degree. C./min., heating the core to a temperature between 0.95 T.sub.c and 150.degree. C. for 1-10 hours in a magnetic field and cooling the core to room temperature. The alloy is a Co-based amorphous metal which includes Si and B and other optional additions. The magnetic field is generally coincidental with the direction of the magnetic path of the core.
Lin discloses a method of improving magnetic properties of a wound core fabricated from amorphous strip metal by applying a force in tension to the loop of the innermost lamination. While the tension force is being applied, the loop is annealed and simultaneously subjected to a magnetic field of predetermined strength. The core can be round or it can have a rectangular shape comprised of spaced-apart legs, an upper yoke, and a lower yoke. An associated electrical coil or coils can be assembled about the core by winding the coil or coils about a section of the core in a conventional manner. Alternatively, one of the core yokes or legs may include a joint to provide access into and around the core for positioning an associated electrical coil or coils. The cores can be annealed in a protective atmosphere such as a vacuum, an inert gas such as argon, or a reducing gas such as a mixture of hydrogen and nitrogen. In the case of METGLAS Alloy 2605 SC, the cores are heated from ambient to a temperature of between 340.degree.-370.degree. C. at a heating rate of 10.degree. C./min, held at that temperature for two hours and cooled to ambient at a cooling rate of 10.degree. C./min. METGLAS Alloy 2605 S-2 is heated to a temperature of between 390.degree.-410.degree. C. for the annealing treatment.
Fluidized beds have been used to heat treat metal workpieces. For instance, it is known to continuously heat treat elongated metal work pieces such as ferrous wires by means of a fluidized bed apparatus, as disclosed in U.S. Pat. No. 4,813,653 ("Piepers"). The apparatus of Piepers includes separate fluidized bed modules, each of which comprises a U-shaped vessel containing inert particles to be fluidized by a fluidizing gas.
The existing methods of annealing amorphous metal alloys such as cores typically require long soak times in a conventional oven, with a protective atmosphere such as nitrogen, to obtain uniform heating throughout the metal. Such a heat cycle, combined with a long cooling step, results in a slow, expensive, and inefficient process. In addition, this slow process results in embrittlement of the amorphous metal due to crystal growth and nucleation of crystals during the annealing treatment.